CARD8: the ‘tripwire’ inside cells that alerts the body to HIV

Researchers at the University of Washington and Fred Hutch found that CARD8, an inflammasome sensor, detects HIV-1 in macrophages during cell-to-cell viral transmissions from infected T cells. 

In previous work, Dr. Michael Emerman, an emeritus professor at Fred Hutch, and Dr. Patrick Mitchell, an assistant professor at the University of Washington, showed that CARD8 acts like a “molecular tripwire” inside cells, detecting the activity of an HIV protease required for viral replication. When activated, CARD8 triggers cell death and inflammatory responses that help limit infection. The team also discovered that human CARD8 can sense HIV protease in a site-specific manner during infection. These findings were previously highlighted by Science Spotlight. In this follow-up study, Emerman and Mitchel focused on whether CARD8 responds during HIV cell-to-cell transmission from T cells to macrophages. Their findings were recently published in eLife.

HIV can infect cells in two main ways: cell-free infection, where free virus particles bind to target cells, and cell-to-cell transmission, where infected cells directly transfer virus to uninfected cells via a temporary viral synapse. The latter mechanism delivers a large influx of virus, resulting in a high multiplicity of infection—the number of viral particles infecting one cell.

To study CARD8 activation, the team co-cultured infected primary T cells with non-infected primary macrophages. The T cells used lacked CARD8, so any activation detected came from the macrophages. They observed increased IL-1β secretion, a hallmark of CARD8 activation, confirming that CARD8 responds during cell-to-cell transmission. Additionally, the team found that CARD8 activationhappens independently of NLRP3, another well-known inflammasome sensor previously linked to HIV detection. These findings extend earlier work showing that CARD8 detects HIV during cell-free infectionand demonstrate that this sensing also occurs during physiologically relevant cell-to-cell spread, a major route of HIV-1 transmission in the body.  

Then, the team also examined different HIV protease variants, including drug-resistant strains that arise during antiretroviral therapy that includes HIV protease inhibitors, and found that these mutations can alter how effectively CARD8 detects infection. Some variants cleaved CARD8 more efficiently, while others were less effective, suggesting that viral evolution under drug pressure could influence inflammasome activation and chronic inflammation in people living with HIV.

The researchers propose that a threshold level of incoming HIV protease is required to trigger CARD8 sensing. In cell-free infection, this threshold can be reached through agents like an DEAE-dextran, which increase viral entry, while in cell-to-cell transmission, the naturally high viral load delivered through the synapse is sufficient. 

Overall, these results suggest that CARD8-driven inflammasome activation in macrophages may contribute to HIV-1 pathogenesis by promoting persistent inflammation and immune activation, and that variation in viral protease, particularly in drug-resistant strains, can modulate this immune response.

The spotlighted work was funded by the National Institutes of Health, the Howard Hughes Medical Institute and the Mallinckrodt Foundation. 

Fred Hutch/University of Washington/Seattle Children’s Cancer Consortium member Dr. Michael Emerman contributed to this research.

Kulsuptrakul J, Emerman M, Mitchell PS. (2025). CARD8 inflammasome activation during HIV-1 cell-to-cell transmission. Elife.